prefix Configuration

The prefix config defaults to the location where node is installed.
On most systems, this is /usr/local. On windows, this is the exact
location of the node.exe binary. On Unix systems, it's one level up,
since node is typically installed at {prefix}/bin/node rather than
{prefix}/node.exe.

When the global flag is set, npm installs things into this prefix.
When it is not set, it uses the root of the current package, or the
current working directory if not in a package already.

Node Modules

Packages are dropped into the node_modules folder under the prefix.
When installing locally, this means that you can
require("packagename") to load its main module, or
require("packagename/lib/path/to/sub/module") to load other modules.

Scoped packages are installed the same way, except they are grouped together
in a sub-folder of the relevant node_modules folder with the name of that
scope prefix by the @ symbol, e.g. npm install @myorg/package would place
the package in {prefix}/node_modules/@myorg/package. See scope(7) for
more details.

If you wish to require() a package, then install it locally.

Executables

When in global mode, executables are linked into {prefix}/bin on Unix,
or directly into {prefix} on Windows.

When in local mode, executables are linked into
./node_modules/.bin so that they can be made available to scripts run
through npm. (For example, so that a test runner will be in the path
when you run npm test.)

Man Pages

When in global mode, man pages are linked into {prefix}/share/man.

When in local mode, man pages are not installed.

Man pages are not installed on Windows systems.

Cache

See npm-cache(1). Cache files are stored in ~/.npm on Posix, or
~/npm-cache on Windows.

This is controlled by the cache configuration param.

Temp Files

Temporary files are stored by default in the folder specified by the
tmp config, which defaults to the TMPDIR, TMP, or TEMP environment
variables, or /tmp on Unix and c:\windows\temp on Windows.

Temp files are given a unique folder under this root for each run of the
program, and are deleted upon successful exit.

More Information

When installing locally, npm first tries to find an appropriate
prefix folder. This is so that npm install foo@1.2.3 will install
to the sensible root of your package, even if you happen to have cded
into some other folder.

Starting at the $PWD, npm will walk up the folder tree checking for a
folder that contains either a package.json file, or a node_modules
folder. If such a thing is found, then that is treated as the effective
"current directory" for the purpose of running npm commands. (This
behavior is inspired by and similar to git's .git-folder seeking
logic when running git commands in a working dir.)

If no package root is found, then the current folder is used.

When you run npm install foo@1.2.3, then the package is loaded into
the cache, and then unpacked into ./node_modules/foo. Then, any of
foo's dependencies are similarly unpacked into
./node_modules/foo/node_modules/....

Any bin files are symlinked to ./node_modules/.bin/, so that they may
be found by npm scripts when necessary.

Global Installation

If the global configuration is set to true, then npm will
install packages "globally".

For global installation, packages are installed roughly the same way,
but using the folders described above.

Cycles, Conflicts, and Folder Parsimony

Cycles are handled using the property of node's module system that it
walks up the directories looking for node_modules folders. So, at every
stage, if a package is already installed in an ancestor node_modules
folder, then it is not installed at the current location.

Consider the case above, where foo -> bar -> baz. Imagine if, in
addition to that, baz depended on bar, so you'd have:
foo -> bar -> baz -> bar -> baz .... However, since the folder
structure is: foo/node_modules/bar/node_modules/baz, there's no need to
put another copy of bar into .../baz/node_modules, since when it calls
require("bar"), it will get the copy that is installed in
foo/node_modules/bar.

This shortcut is only used if the exact same
version would be installed in multiple nested node_modules folders. It
is still possible to have a/node_modules/b/node_modules/a if the two
"a" packages are different versions. However, without repeating the
exact same package multiple times, an infinite regress will always be
prevented.

Another optimization can be made by installing dependencies at the
highest level possible, below the localized "target" folder.

Since foo depends directly on bar@1.2.3 and baz@1.2.3, those are
installed in foo's node_modules folder.

Even though the latest copy of blerg is 1.3.7, foo has a specific
dependency on version 1.2.5. So, that gets installed at [A]. Since the
parent installation of blerg satisfies bar's dependency on blerg@1.x,
it does not install another copy under [B].

Bar [B] also has dependencies on baz and asdf, so those are installed in
bar's node_modules folder. Because it depends on baz@2.x, it cannot
re-use the baz@1.2.3 installed in the parent node_modules folder [D],
and must install its own copy [C].

Underneath bar, the baz -> quux -> bar dependency creates a cycle.
However, because bar is already in quux's ancestry [B], it does not
unpack another copy of bar into that folder.

Underneath foo -> baz [D], quux's [E] folder tree is empty, because its
dependency on bar is satisfied by the parent folder copy installed at [B].

For a graphical breakdown of what is installed where, use npm ls.

Publishing

Upon publishing, npm will look in the node_modules folder. If any of
the items there are not in the bundledDependencies array, then they will
not be included in the package tarball.

This allows a package maintainer to install all of their dependencies
(and dev dependencies) locally, but only re-publish those items that
cannot be found elsewhere. See package.json(5) for more information.